by finding the optimal depth assuming that we have an ideal engine for
turning heat into electricity, and that drilling to any depth is free.

For the temperature profile shown in figure 16.4, I calculated that the
optimal depth is about 15 km. Under these conditions, an ideal heat engine
would deliver 17 mW/m2. At the world population density of 43 people
per square km, that’s 10 kWh per person per day, if all land area were
used. In the UK, the population density is 5 times greater, so wide-scale
geothermal power of this sustainable-forever variety could offer at most2 kWh per person per day.

This is the sustainable-forever figure, ignoring hot spots, assuming per-
fect power stations, assuming every square metre of continent is exploited,
and assuming that drilling is free. And that it is possible to drill 15-km deep
holes.

Geothermal power as mining

The other geothermal strategy is to treat the heat as a resource to be mined.
In “enhanced geothermal extraction” from hot dry rocks (figure 16.5), we
first drill down to a depth of 5 or 10 km, and fracture the rocks by pumping
in water. (This step may create earthquakes, which don’t go down well
with the locals.) Then we drill a second well into the fracture zone. Then
we pump water down one well and extract superheated water or steam
from the other. This steam can be used to make electricity or to deliver
heat. What’s the hot dry rock resource of the UK? Sadly, Britain is not well
endowed. Most of the hot rocks are concentrated in Cornwall, where some
geothermal experiments were carried out in 1985 in a research facility at
Rosemanowes, now closed. Consultants assessing these experiments con-
cluded that “generation of electrical power from hot dry rock was unlikely
to be technically or commercially viable in Cornwall, or elsewhere in the
UK, in the short or medium term.” Nonetheless, what is the resource? The
biggest estimate of the hot dry rock resource in the UK is a total energy of
130 000 TWh, which, according to the consultants, could conceivably con-
tribute 1.1 kWh per day per person of electricity for about 800 years.

Other places in the world have more promising hot dry rocks, so if you
want to know the geothermal answers for other countries, be sure to ask a
local. But sadly for Britain, geothermal will only ever play a tiny part.

Doesn’t Southampton use geothermal energy already? How
much does that deliver?

Yes, Southampton Geothermal District Heating Scheme was, in 2004 at
least, the only geothermal heating scheme in the UK. It provides the city
with a supply of hot water. The geothermal well is part of a combined heat,
power, and cooling system that delivers hot and chilled water to customers,
and sells electricity to the grid. Geothermal energy contributes about 15%
of the 70 GWh of heat per year delivered by this system. The population

Figure 16.5. Enhanced geothermal extraction from hot dry rock. One well is drilled and pressurized to create fractures. A second well is drilled into the far side of the fracture zone. Then cold water is pumped down one well and heated water (indeed, steam) is sucked up the other.